Liquid-metal feed system for feeding propellant to an ion thrustor

ABSTRACT

A reservoir of liquid metal is connected to an ion thrustor for a supply of ionizable material thereto. The reservoir is in the form of a reservoir cylinder having a reservoir piston therein. An associated drive cylinder contains a drive piston connected to the reservoir piston. A liquid in equilibrium with its vapor is positioned in the drive cylinder, and as heat is added, the pistons move to deliver liquid metal to the thrustor. The amount of heat supplied to the drive cylinder is preferably a function of the liquid-metal requirements of the ion thrustor.

United States Patent [111 77 4 [72] Inventor Harry J. King 2,548,7084/1951 Dickey 60/25 3811 38 Park, Calil- 3,067,572 12/ 1962Baumgartner... 60/25 [21 Appl. No. 746,114 3,096,456 7/1963 Shelton etal. 60/202X [22] Filed July 19, 1968 3,097,483 7/1963 Bixson et al.60/39.48X Division of Ser. No. 602,664, Dec. 19, 1966, 3,256,686 6/ 1966Lindberg 60/25 Pat. No. 3,443,383 3,291,054 12/ 1966 McKenzie 60/25X[45] Patented May 4, 1971 3,427,808 2/1969 Butcher 60/39.48X [73]Asslgnee ompany Primary Examiner-Carlton R. Croyle Attorneys-James K.Haskell and Allen A. Dicke, Jr.

[54] LIQUID-METAL FEED SYSTEM FOR FEEDING PROPELLANT TO AN ION THRUSTORzclmms l Drawmg ABSTRACT: A reservoir of liquid metal is connected to an[52] U.S. CI 60/202, ion thru torfor a supply of ionizable materialthereto. The reservoir is in the form of a reservoir cylinder having areser- [51] Int. Cl. F03h 5/00 ir i ton therein, An associated drivecylinder contains a [50] Fleld of Search 60/25, 202, driv i ton onnectedto the reservoir piston. A liquid in 103/43, 48, 50 l equilibrium withits vapor is positioned in the drive cylinder, and as heat is added, thepistons move to deliver liquid metal [56] References C'ted to thethrustor. The amount of heat supplied to the drive UNITED STATES PATENTScylinder is preferably a function of the liquid-metal require- 1,856,5865/1932 Persons 60/25 ments of the ion thrustor. I

Ref,

i Thruster B J OWE? I DQDHIIQDDUDUUULI l I 7' l PATENVVTEDMAYAIHH v3577.734

Thruster B Harry J. King,

INVENTOR.

ALLEN A. DICKE, Jr.,

AGENT.

LIQU ID-METAL FEED SYSTEM FOR FEEDING PROPELLANT TO AN ION THRUSTORCROSS REFERENCE This invention is divided out of patent application Ser.No. 602,664, now US. Pat. No. 3,443,383, filed Dec. 19, 1966, entitledFluid Feed System.

BACKGROUND This invention relates primarily to a fluid feed system, andmore particularly to methods and apparatus for controllably feedingpropellant from a reservoir to an ion engine.

Prior to the present invention, liquid mercury propellant was fed from amercury reservoir to a mercury ion thrustor by using a high pressure gasfrom a high pressure gas bottle whose pressure was controlled by amechanical regulator in the feedline between the bottle and thereservoir.

The reservoir was a spherically shaped container with a flexible metaldiaphragm separating the mercury and the gas. This was a one-shot systemsince the reservoir could not be refilled. The diaphragm was supposed toturn itself inside-out as the mercury was used up. However, thediaphragm often ruptured and also unfolded in such a way as to catchquantities of mar cury in small pockets so that all of the mercury wasnot available for use. Flexible elastomeric diaphragms were found to besubject to most of the same disadvantages.

Further, because such regulators possess a small but finite leak rateand because gas must be vented to reduce system pressure, large amountsof gas would be consumed over the long hours) required life of an ionthrustor feed system. Rough calculation indicates that the high pressuregas bottle required would be larger than the mercury reservoir itself.

SUMMARY As an aid in the understanding of this invention, it can bestated in essentially summary form that it is directed to a feed systemfor feeding ionizable material to an ion engine, the feed meanscomprising a reservoir cylinder having a piston therein, and a drivecylinder having a piston therein. The pistons are connected so thatmotion on the drive piston causes motion of the reservoir piston. Thepressure on the drive piston is controlled by thermally adjusting thevapor pressure of the liquidvapor above the drive piston to control theflow rate out of the reservoir cylinder to the ion engine. Such controlcan be in accordance with the requirements of ion engine for ionizablematerial.

It is therefore, a primary object of the present invention to provide afluid feed system which is not subject to the above disadvantagesinherent in previous feed systems. It is a further object of theinvention to provide a fluid feed system which does not require a bulkygas storage bottle or a mechanical regulator. It is another object ofthe invention to provide a fluid feed system which does not require theventing of gas to reduce pressure on the fluid. It is still a furtherobject of the invention to provide a fluid feed system in which the onlymoving part is a slow moving piston which rides on low stick bearings.It is another object of the inventionto provide a liquid mercury feedsystem for use which provides smooth, high pressure, mercury feed at lowflow rates. It is another object of the invention to provide apropulsion system comprising a liquid mercury cathode and liquid mercurystorage and flow control means. It is another object of the invention toprovide a fluid feed system in which essentially all of the fluid isavailable for use.

These and other objects and advantages of the present invention will bemore fully understood by reference to the following detailed descriptionwhen read in conjunction with the attached drawing.

DESCRIPTION OF THE DRAWING Referring to the drawing, the FIGURE shows aliquid mercury reservoir 2 adapted to contain a quantity of mercury in achamber 4 therein. In the preferred embodiment, the mercury in chamber 4is fed under pressure through a conduit 5 to a thrustor 7 (such as aKaufman engine such as is shown in Pat. No. 3,l56,090). Other elementsof such a system such as valves, a vaporizer and an isolator, areconventional and form no part of this invention and need not bedescribed in detail herein. However, in the embodiment described herein,the control for the propellant feed includes a closed loop in whichchanges in the ion beam, as signalled by the beam current I provide thecontrol signal to vary the propellant feed rate. It is noted that thissystem is applicable to other liquids than mercury and also to fluidsand for other applications than for feeding propellant to ion engines.

Attached to the reservoir 2 is a pressurizer section 6 which contains apiston and cylinder arrangement comprising a stationary cylinder 10inside of which is mounted a piston 8 comprising a piston head 9attached to a bellows 11. A liquid is contained in the chamber 12between the piston 8 and the cylinder 10. In the preferred embodiment,this liquid is water in equilibrium with its saturated vapor. The vaporpressure of the liquid is a unique function of the temperature of thecylinder 10. The temperature of the liquid in the chamber 12 iscontrolled by means of a heater 14, which comprises a heater filamenthelically wound upon and in contact with the outer surface of thecylinder 10. As the volume of the cylinder 12 increases due to motion ofthe piston 8, the liquid in the chamber 12 evaporates so as to maintainthe saturated vapor condition.

The pressurizer 6 and the reservoir 2 are thermally isolated from eachother and the force from the pressure of vapor in the chamber 12 istransmitted from the pressurizer 6 to the reservoir 2 through a push rod16 of low thermal conductivity material. A stainless steel shield 13aids in insulating the pressurizer 6 from the reservoir 2. Thermalisolation is desired so that the temperature of the relatively low massof the pressurizer may be rapidly changed by small amounts of inputpower. With tight thermal coupling between the pressurizer 6 and thereservoir 2 it would be necessary to change the temperature of both thepressurizer 6 and the mercury reservoir 2, which could contain severalhundred pounds of mercury. In such a case, this system would haveintolerably slow response.

The mercury in the chamber 4 is contained by means of a seal 18 betweenthe side of the reservoir 2 and a piston 20.

The side of the reservoir 2 and the piston 20 may be considered anotherpiston-cylinder configuration similar to the piston-cylinderconfiguration in the pressurizer 6. The seal 18 is preferably of therolling diaphragm type to reduce friction and contamination. If anall-metal system was required for high temperature or high pressureoperation, the seal 18 can equally well be made with a metal bellowswhich allows a long stroke. The piston 20 provided with a cylindricalskirt 22 to prevent the diaphragm 18 from bulging as the mercury isexpelled and to insure that all of the mercury is available for use inthe system.

' In actual operation, the embodiment shown has been used to supplyliquid mercury flow rates form 0 to l cc./hr. to a 0 liquid-mercurycathode while it was operating in an ion thrustor and to supply liquidmercury at constant pressure (about 7 p.s.i.) to a vaporizer operatingas part of a vapor feed system for an ion thrustor using an oxidecathode. Closed loop operation using the ion beam current as the inputsignal has been demonstrated to be useful with a liquid-mercurythrustor. This closed loop operation is schematically shown in thedrawing.

The drawing shows ion thrustor 7 connected to the chamber 4 by means ofthe conduit 5. The thrustor 7 can be any of the known types of ionthrustors, such as the Kaufman engine, and can employ, for example, aliquid-mercury cathode or a thermionic cathode.

A control circuit is illustrated for use with a liquid-metal cathode,for which it is desirous to vary the pressure on the mercury in thereservoir in order to vary the flow of propellant to the thrustor. Theion beam current 1,, is passed through a fixed resistor 26 to develop avoltage. This voltage is compared to a reference voltage 27 by means ofa differential amplifier 28. The resulting difference signal isamplified by a power amplifier 29 such as a magnetic amplifier and asilicon controlled rectifier. The amplified difference signal is used bya power controller 30 to adjust and control the power going to theheater 14 of the pressurizer 6. This control loop controls thetemperature of the pressurizer, hence the driving pressure, andultimately the mercury flow rate and the ion beam current. In otherapplications of this invention, the control signal to the powercontroller 30 can originate from a fluid flow measuring devicepositioned in the outlet conduit 5.

This invention having been described'in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

I claim:

1. Apparatus for controllably feeding a fluid material comprising:

an enclosed reservoir adapted to contain a quantity of fluid material,said enclosed reservoir comprising a first piston slidably mounted in afirst cylinder so that movement of said first piston expels fluidmaterial from said first cylinder;

means for receiving the fluid material;

conduit means connected between said reservoir and said receiving meansfor providing fluid communication therebetween;

a pressurizer having an enclosed pressurizer chamber adapted to containa quantity of liquid in equilibrium with its vapor, means formaintaining said pressurizer in thermal isolation;

said pressurizer comprising a second cylinder closed at one end andprovided with a second movable piston slideably mounted in said secondcylinder in sealing engagement with the walls thereof to define apressurizing chamber therein adapted to contain a quantity of liquid inequilibrium with its vapor, said first piston being connected to bemoved by said second piston;

heater means associated with said second cylinder for heating the liquidin said pressurizing chamber for controlling the temperature of theliquid and vapor in said chamber; and

means responsive to the rate of flow of said fluid material from saidreservoir for controllably energizing said heater means.

2. Apparatus for controlling the flow of liquid mercury from a reservoirto an ion thrustor comprising:

a reservoir adapted to contain a quantity of mercury;

a piston slideably mounted in said reservoir for varying the volumethereof;

a rolling diaphragm sealing means for sealing said piston in saidreservoir;

an ion thrustor;

conductor means connected between said reservoir and said thrustor forproviding fluid communication therebetween;

2. Apparatus for controlling the flow of liquid mercury from a reservoirto an ion thrustor comprising: a reservoir adapted to contain a quantityof mercury; a piston slideably mounted in said reservoir for varying thevolume thereof; a rolling diaphragm sealing means for sealing saidpiston in said reservoir; an ion thrustor; conductor means connectedbetween said reservoir and said thrustor for providing fluidcommunication therebetween; a pressurizer comprising a piston-cylinderarrangement defining a pressurizing chamber adapted to contain aquantity of liquid in equilibrium with its vapor; heater meansassociated with said pressurizer; and means responsive to the rate offlow of mercury from said reservoir for energizing said heater means.